The invention relates to a two-stroke internal-combustion engine in which a selective control of the charge introduced in the combustion chamber through one or several inlet ports is achieved, in dependence upon the working conditions of the engine.
According to the invention, the inlet ducts are "throttled" differently with respect to each other in dependence upon to the working conditions of the engine.
The selective control of the introduction of the charge according to the invention is preferably achieved on two-stroke engines of the cross-scavenging type as defined hereafter.
The control can be applied in case the charge introduced contains no fuel, the fuel being then introduced through a separate inlet.
The pressure source used for the supply of air or of carbureted mixture in the cylinder can include a pressure source outside the engine such as a compressor, a blower or any other volume under pressure, or of an inner source such as the pump housing.
Two-stroke engines conventionally comprise at least one cylinder in the upper part thereof and a lower part, also called pump housing, in view of the main function thereof, which provides the introduction of fresh gases into the cylinder by one or more specific ducts known as "transfer ducts". Transfer ports or openings in the cylinder allow this introduction.
The piston which moves in a reciprocating manner in the cylinder provides the suction in the housing and the compression of the fresh gases in the cylinder. The fresh gases are generally introduced at the level of the pump housing through an opening, for example, provided with a check valve. The motion of the piston towards the upper part of the cylinder brings about the aspiration of fresh gases into the pump-housing; whereas, the opposite motion of the piston compresses these gases which, when the transfer ports are freed by the piston, are then injected under a given pressure into the cylinder, which generates therein a scavenging of fresh gases that drive waste gases away.
The waste gases can escape through openings known as exhaust openings provided in the cylinder, slightly offset in height with respect to the transfer ports. What is called "fresh gas" generaly relates to a mixture of air, carbureted or not.
One of the most serious problems with two-stroke engines supplied with a fuel-air mixture is that the intake of a subsequent cycle occurs at the same time as the exhaust of the previous cycle, so that a large part of the mixture fed to the cylinder directly goes back to the exhaust, without the combustion of the hydrocarbons.
The atmospheric pollution resulting from this phenomenon is considerable and is about 10 to 20 times as high as the pollution generated by a four-stroke engine. Also, the fuel consumptions is high and may, for example, be 50 to 100% greater than the fuel consumptions of a four-stroke engine.
Considerable research has been carried out in order to decrease the losses of unburned fuel through the exhaust.
The basic concept of most of the suggested improvements consists in delaying the introduction of the fuel which then occurs, for example, when the exhaust port is almost closed. But, if the introduction of the fuel is delayed, it will have to be introduced and vaporized within a very short time (about 2 to 3 milliseconds at high engine speed), which may pose problems.
In two-stroke engines of the "liquid direct fuel injection" type, the fresh gas charge contains no fuel. It can be compressed, either by an outer mechanical compressor, or by a compression source provided, for example, by the pump housing itself.
The fuel is then directly injected into the combustion chamber under high pressure, a pressure which is generally higher than 30 bars.
Another type of two-stroke engine capable of delaying the introduction of the fuel is based on the principle of the air-blast injection of the mixture. Compressed air is used in this case for allowing to spray and to vaporize very rapidly the fuel in the cylinder.
There are several ways of achieving the source of pressure. For example, U.S. Pat. No. 4,693,224 shows the use of a specific compression chamber intended to contain a given amount of fuel under pressure and to inject this mixture into the combustion chamber.
In a different way, French patent application FR-2,496,7571 proposes an air-blast injection of fuel in the cylinder by using the pressure of the fresh gases inside the pump housing. A means for proportioning the liquid fuel is therefore directly linked to the transfer duct coming from the pump housing. The air compressed in the pump housing and sent towards the proportioning means through at least one specific duct provides in this case the spraying of the fuel within a very short time. The fuel droplets, very thin, are immediately vaporized into the combustion chamber.
The result of all that has been written so far is that the way the charge is introduced and then directed in the combustion chamber of a two-stroke engine is very important, and must therefore be achieved in an optimum way, with a control as rigorous as possible. In other words, the engine efficiency depends to a large extent on the spraying and the injection of the fuel into the cylinder and on the internal aerodynamics responsible for the mixing of this fuel with the fresh air.
In FR-2,649,157 one or more parts for restricting the flow of the fresh gases allowed to pass into the cylinder are provided inside the transfer duct(s) and close to the cylinder.